People use smartphone apps to make their lives easier in many ways, from finding a nearby restaurant to taking notes to getting directions to their destination, but would you trust a smartphone app to help you avoid unwanted pregnancies?

The app 'Natural Cycles' promises to help prevent unwanted pregnancies and it's catching on in a big way in the UK and Europe, here's how it works:

‘Natural Cycles’—yet to attain FDA approval—was founded by husband-wife team Dr Raoul Scherwitzl and Dr Elina Berglund. It scans the body’s temperature during the menstrual cycle to identify if it is safe for the female user to engage in unprotected sex.

Days that are safe are marked in green on the app’s calendar. Otherwise, a red color warns the user to use protection. Last year, Engadget reported that a study conducted by the app revealed, “After testing 22,785 women throughout 224,563 menstrual cycles, the startup found the app provided 99 percent contraceptive effectiveness if used perfectly. If used ‘typically’, the app was 93 percent effective. The contraceptive pill, meanwhile, is 91 percent effective.”

Hundreds of trusting Swedish women used 'Natural Cycles' believing it would work as advertised, and yet in at least 37 cases the app failed and the users wound up pregnant.

Now the app is being reviewed by the Swedish Medical Products Agency, a regulatory agency for medical devices, but the creators of the app insist the amount of unwanted pregnancies is quite normal:

“To have 37 unwanted pregnancies out of the 668 mentioned in this study at Södersjukhuset means that 5.5 per cent of women who stated they used Natural Cycles also had an unwanted pregnancy. This is in line with what we communicate as the risk of unwanted pregnancy with typical use, and which is comparable to other types of contraception.”

“As our user base increases, so will the amount of unintended pregnancies coming from Natural Cycles app users, which is an inevitable reality.”

“Today there is a big trend for women to move away from hormonal contraception, and Natural Cycles can provide a helpful option for these women. We therefore expect, in fact, overall to decrease the unwanted pregnancy rates because we’re increasing contraceptive choice, and see that many of our users are women that have not used any type of contraception before.”